Why is 日本 called にっぽん？

Sometimes you will notice that 日本 is called にっぽん (Nippon)? Why is that so?

Actually the original name for 日本 was called 日本国、in AD 701, 日本 was enacted to be used.

In government agencies, it’s quite frequent that you will see にっぽん instead of 日本 being used. Apart from that, during sports competition with foreign countries, Japanese will use にっぽん when cheering for their home team.